1. Field of the Invention
The present invention relates to the use of formaldehyde-free aqueous binders having a broad molecular weight distribution for substrates, said binders comprising    (A) from 0 to 100% by weight of an ethylenically unsaturated acid anhydride or ethylenically unsaturated dicarboxylic acid whose carboxylic acid groups can form an anhydride group, or mixtures thereof,    (B) from 100 to 0% by weight of an ethylenically unsaturated compound,    (C) at least one polyfunctional crosslinker or mixtures thereof,the polymers of A) and B) obtained by free-radical addition polymerization, when classified in a coordinate system by way of their average molecular weight Mw and their polydispersity, being situated in the area above a straight line which is defined by the linear equation y=1.25x+20 000 and has been shifted in y direction parallelwise by at least +3 000, the x axis denoting the weight-average molecular weight and the y axis the polydispersity times 10 000.
2. Description of the Background
The invention further relates to the binders themselves and also to their use for moldings, mats or boards, for example, in particular for fibrous and particulate substrates such as fiber webs, glass fibers, rockwool, reclaimed cotton, natural fibers or synthetic fibers.
The consolidation of sheetlike fibrous structures, or fiber webs, is effected, for example, purely mechanically by needling or water jet consolidation of a wet-laid or air-laid web or by chemical consolidation of the webs with a polymeric binder. The binder is generally applied by impregnating, spraying or coating. To enhance the wet strength and heat resistance of the webs use is frequently made of binders comprising crosslinkers which give off formaldehyde. Alternatives to existing binders are sought by the skilled worker in order to avoid formaldehyde emissions.
U.S. Pat. No. 6,221,973 discloses a formaldehyde-free, crosslinkable aqueous composition comprising a polyacid, a polyol, and a phosphorus-containing reaction accelerant for use as binders for heat-resistant nonwovens, e.g., glass fibers.
EP 990 727 discloses binders for mineral fibers, comprising a low molecular mass polycarboxy polymer and a polyol, the pH of the binder being not greater than 3.5.
U.S. Pat. No. 5,932,665 discloses binders based on polycarboxy polymer, this system being curable at lower temperatures than in the case of comparable systems composed of homopolyacrylic acids, by setting of the molecular weight and of the copolymer composition.
EP 882 074 describes formaldehyde-free aqueous binders comprising an ethylenically unsaturated acid anhydride or an ethylenically unsaturated dicarboxylic acid and an alkanolamine as coating materials, impregnants and binders for fiber webs.
The binders described to date in the state of the art use either low or high molecular mass polycarboxylic acids, i.e., polycarboxylic acids having a molecular weight distribution which is typical of free-radical addition polymerizations. Substrates produced using high molecular mass binders have a high strength, a quality determined by means, for example, of tensile strength measurements. Low molecular mass binders based on polycarboxylic acid are distributed effectively on the substrates, since they possess high fluidities (as measured by dynamic mechanical analysis, determination of the dynamic storage modulus G′), but have deficiencies in the resulting strength properties.